Feature recognition techniques

ABSTRACT

A feature recognition system for extracting transient signal information from a signal-like noise background by separating a time domain signal into a plurality of frequency-time domain signals by frequency selective filters. Each of the filtered outputs is converted to unidirectional signals and then displayed on a chart recorder from which the presence of a signal is determined on the basis of one or more of three criteria. The first criteria is maximum energy in a predetermined one of the selected frequencies; the second criteria is the time convergence of the time-tracings of peaks and troughs of the unidirectional signal lobes at the higher frequencies; and the third criteria is the difference in shape factors for sgnal information as opposed to noise information.

O Umted States Patent 1191 1111 3,737,842 Bobrin 1451 June 5, 1973 s41FEATURE RECOGNITION 3,102,928 9/1963 Schroeder ....324 77 E x TECHNIQUES3,196,212 7/1965 l-lorwitz..... ....324/77 E x 3,215,934 11/1965 Sallen....324/77 E X Inventor: Marshall B0brm, Phlladelphla, 3,140,710 7/1964Glassneret al ..324/77 E x Pa. Primary Examiner-Richard A. Farley [73]Assignee. The United States of America as represented by the Secretaryof the Attorney-G. J. Rubens and Henry Hansen N W h' t D.C.

as mg 57 ABSTRACT [22] Filed: Mar. 30, 1966 A feature recognition systemfor extracting transient ['21 1 Appl. No.: 540,148 signal informationfrom a signal-like noise background by separating a time domain signalinto a plurality of frequency-time domain signals by frequency selective[52] U.S.Cl. ..340/4 R, 324lgzl27tlli filters Each of the filteredoutputs is Converted to unidirectional si nals and then dis la ed on achart s1 rm (:1 G01r33/02 g P 58 8 77 recorder from which the presenceof a signal is deter- 1 0 care 340 1 M mined on the basis of one or moreof three criteria. l The first criteria is maximum energy in apredetermined one of the selected frequencies; the second [56] Refermcescued criteria is the time convergence of the time-tracings of UNITEDTATES PATENT peaks and troughs of the unidirectional signal lobes at S Sthe higher frequencies; and the third criteria is the dif- 2,632,8843/1953 Murphy ..324/8 X ference in shape factors for sgnal informationas op- 2,705,742 4/1955 Miller .324/77 E X posed to noise information,2,967,998 1/1961 Hurvitz ....324/77 E 3,051,897 8/1962 Peterson et a1...324/77 E 1 Claim, 7 Drawing Figures SUBMARINE DErE igm SIGNAL GEOLOGICNOISE EVENT 1256620571 22 1 2 I -13 QHANNE! 1 I P. R E STIEIER ooaacps I-14 I CHANNEL 2 I H a m s-15R f 0,075cps l -15 l QHANNEL I H ggliilER If 0.12cps I his CHANNL FILLEZQTIBFIER I t 0.1900 I N 1 QHANNEL, 5 IFILLEERCT?FIER 0.30cps I hue M QHANNEL 6 FILTER a RECTIFIER l f,,=048cps 1 mum -19 i FILTER B MT RECTIFIER I l T I 1 I (a ojscps 0 1O 2OBOrIMGEO rsggofllfsl 7O 8O 90 100 Patented June 5, 1973 MAD DETECTOR ANDRECORDER CHANNEL I FILTER 8 3 Sheets-Sheet 1 SUBMARINE SIGNAL GEOLOGICNOISE EVENT RECTIFIER oms p CHANNEL 2 FILTER a RECTIFIER f 0.075cpsQHAQNEI.

FILTER 8:

RECTIFIER f 0.12cps CHANNEL 4 FILTER 8 RECTIFIER CHANNEL 5 FILTER &

RECTIFIER f 0.3OCPS FILTER 8 RECTIFIER f0 0.4BCPS QHANNEL 7 FILTER 8RECTIFIER f 0.75cps I IO 20 3O I I I I I 7O 8O INVENTOR.

MARSHALL C. BOBRIN BY IMIM A T TORNE Y RELATIVE INTENSITY CENTRAL LOBEAMPLITUDE 2 3 4 6 7 n1. rm, CHANNELS F ig.

Patented June 5, 1973 3,737,842

5 Sheets-Sheet 2 I F25 I GEOLOGIC NOISE EVENT L ZZ SUBMARINE SIGNAL I II V 1 l I 0 1O 2O 3O 0 5O 6O 7O 8O TIME (SECONDS) 2 INVENTOR. MARSHALLC. BOBRIN F11. TE? CHANNELS A T TORNE Y Patented June 5, 1973 3,737,842

3 Sheets-Sheet 3 MAD necono 0F GEOLOGIC NOISE EVENT CH4 NNE L Ff. 4 3.Fig. 5

MAD SUB SIGNAL F IL FER CHI/ NFL INVENTOR.

MARSHALL C. BOBRIN BY Mm ATTORNEY i under the surface of the ocean.These devices produce.

an output signal which is a representation of a variation in the earthsmagnetic field due to the presence of a submarine or other metallicmaterial. The operation of these devices is described in MagneticAirborne Detectors, Volume No. of Summary Technical Report of Division6, National Defense Research Committee, 1946.

In general, the MAD equipment is located in an aircraft which is flownabove the surface of the water in an area in which the presence of asubmarine is suspected. The principal problem of submarine detection inwhich the MAD techniques are employed is that at high altitudes andlarge slant ranges the signal amplitude from a submarine greatlydecreases (as the cube root of the distance) from the detection system.Accordingly, under these conditions, the signals virtually disappearinto the noise background which has characteristics very similar to thesignals.

Prior art systems employed a human operator to discem actual submarinesignals from the surrounding noise by depending upon the operatorsability to recognize characteristics or patterns in the displayed waveshapes from previously memorized patterns which were probably acquiredin a non-noise environment. At low altitudes and short slant ranges theMAD signals are of sufficient amplitude to discriminate between realsignals and the background noise and even in areas of high metallicbackground, the period of the submarine signal is sufficiently shorterthan that of the background noise so that an operator may achieve a highdegree of accuracy in recognizingsubmarines; however, at high altitudes,it becomes exceedingly difficult for an operator to distinguishsubmarine signals from the background noise.

It has been found that certain signals, and in particular submarinesignals, exhibit a particular frequency characteristic or signatureindependent of its amplitude, and a spectral analysis of the waveformcan often differentiate between these signals and background transientnoise. The present invention, accordingly, fulfills this need byproviding a spectral analysis of signal and noise information todetermine the presence of signal information in a signal-like noiseenvironment.

The general purpose of the present invention is therefore to provide adevice which can extract transient signal information from backgroundtransient noise even in a high transient noise condition.

An object of the present invention is to provide a device that separatestransient signal information from transient noise signals.

Another object of the invention is to provide a device for extractingthe characteristics uniquely possessed by submarine signal informationand not by the signal-like background noise and displaying thisinformation for analysis.

A further object of the invention is to simplify the analysis ofsubmarine signatures by providing a device which will allow an operatorto make a decision on the presence or absence of a submarine signal.

Still a further object of the invention is to provide distinguishingcharacteristics between signal and noise information so that a decisioncan be made on the presence or absence of a signal even in a high noiseenvironment.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the follow ing detailed description when considered inconnection with the accompanying drawings wherein:

FIG. 1 illustrates a specific embodiment of the invention and therectified filter outputs;

FIG. 2 illustrates typical transient signal and noise information from aMAD recorder;

FIG. 3 illustrates a three dimensional view of FIGS. 1 and 2;

FIGS. 4 and 5 show typical time tracings for lobe amplitudes as afunction of filter frequency; and

FIGS. 6a and 6b illustrate the central lobe amplitudes as a function offilter frequency for the time tracings of FIGS. 4 and 5.

Briefly, the invention provides a system for feature recognition oftransient signal information based on the separation of the variousfrequency components that constitute a particular signal signature fromthose of transient noise signals which have characteristics similar tothe signal in the time domain but different in the frequency domain.

Referring now to the drawings, there is shown in FIG. 1 a specificembodiment of the invention wherein a MAD detector and recorder 11 areelectrically connected together to detect and record transienttimedomain signals of submarine signatures and geologic noise events.The output of the recorder is electrically connected to the inputs of afilter bank 12 having channels 1 through 7 and generally referred to as13 through 19. Each channel consists of a constant low-Q bandpass filterwith a different center frequency varying from 0.048 cycles per secondto 0.75 cycles per second.

The function of each filter is to transform the transient time-domainsignal (from the MAD record) to the frequency-time domain; that is, timewaveforms of various filter frequencies. The output of each filter iselectrically coupled to a full-wave rectifier which provides an outputto a chart recorder 21 of a rectified signal for each filter, as shownin FIG. 1.

While a full-wave rectifier is employed in the specific embodiment toprovide unidirectional signals, it should be noted that a squaringcircuit which squares the amplitude of each cycle would function equallywell.

Assume for the moment that the waveform 25 at the output of the MADdetector and recorder 1 1, as shown in FIG. 2, contains a submarinesignal signature among random transient geologic noise and that thisinformation is unknown to an operator attempting to determine if in facta submarine is present. The first criteria for determining the presenceof a submarine signal is the frequency at which maximum signal energy isobtained. This is accomplished by passing this record into the filterbank 12, as illustrated in FIG. 1. Each filter output signal from thelower frequency filters is found to contain three prominent lobes,either to the left or right of a line 22, with the central lobe oneither side of the line being the most prominent. Considering for themoment only the central lobes in each channel to the left of line 22, itcan then be seen that the maximum output amplitude occurs in filterchannel 4. This maximum amplitude, being the highest of all the centrallobes in all channels to the left of line 22 is referred to as thesupremum.

From past experience in which known submarine signature information hasbeen artifically combined with geologic transient noise in the recordedinformation, it has been found that submarine signals possess suprema inchannels 4 or S, or both, while on the other hand, signal-like geologicnoise events almost always give rise to suprema in channel 2 at a latertime. For the particular embodiment illustrated herein, submarine signalinformation exists in only the first forty seconds of time, hence line22 divides the signal information from the noise information. In actualsystem operation, the submarine signal and geologic noise may, in fact,be superimposed or interchanged in time; however, the frequency at whichthe maximum energy occurs for the submarine signal and for the noiseremains unaffected. Accordingly, regardless of the signal and noise timerelationship, there is a clear and distinct spectral difference betweensubmarine signals and their signal-like background noise. FIG. 1illustrates this difference by showing a suprema in channel 4 and amaximum in channel 2 displaced in time from the suprema in channel 4.

Accordingly, the first of three unique features which provide evidencefor making a decision on the presence or absence of a submarine has beendisclosed; that is, the frequency at which the energy maximum for thesubmarine signal occurs is different from the frequency at which theenergy maximum for the noise occurs.

The second distinguishing characteristic of a submarine signature in anoise environment is that of the time tracing of the peaks (maxima ofthe lobes) and troughs (minima) for the submarine signal differ fromthose of the signal-like background noise. This feature is bestillustrated with reference to FIG. 3 which shows a three-dimensionalplot of the information displayed in FIG. 1 and a two-dimensional plotof the MAD record of FIG. 2. The X-axis in the horizontal plane is thetime axis for both the two-dimensional MAD record and thethree-dimensional time-frequency amplitude display of the MAD record.The Y-axis in the horizontal plane is the frequency axis and the Z-axisin the vertical plane is the axis along which the output amplitudes ofthe filters are plotted. Each vertical plane therefore represents afilter output whose amplitude changes with time.

Since each filter is tuned to a different center frequency, each filterhas a different response time, (the greater response times correspindingto the lower frequencies); accordingly, the lobes in each filter have aspecific position in time. The larger the response time of a filter, themore its output signal is delayed. As expected, the plots of FIGS. 1 and3 illustrate that the greater time shifts occur at the lowerfrequencies. From FIG. 1 it can be seen that the peak of the centrallobe for the submarine signal occurs at 28.5 seconds from the origin inchannel I and 25.5 seconds from the origin in channel 2.

The different times of occurrence of each lobe peak and trough may beplotted as a function of filter fre quency, resulting in a time-tracingof the central lobes throughout the filters. FIG. 3 illustrates such atimetracing on the three-dimensional plot, wherein a dashed line 26connects a vertical projection of the peaks of the lobes in each filteron the frequency-time plane and a solid line 27 connects a verticalprojection of the troughs between the lobes on the frequency-time plane.Similar time-tracings can be made for the peaks of the lobes to the leftand to the right of the central lobe 24 and time-tracings may also bemade for the troughs between the lobes, thereby forming timetracings asshown in the lower portions of FIGS. 4 and 5 now to be described.

FIGS. 4 and 5 illustrate MAD submarine signals 31 and 32 and geologicnoise signals 41, respectively. Signals 31, 32 and 41 are typical MADsubmarine and noise signals as they would appear on a conventional MADrecording as illustrated in FIGS. 2 and 3. The lower graphs in FIGS. 4and 5, represented by the horizontal lines numbered 1 through 7, depictthe timetracings for the seven filter output channels with the dashedlines indicating the time-tracings of the peaks of the lobes and thesolid lines indicating the time-tracings of the troughs.

The time-tracings in FIG. 4 for the MAD submarine signal 31 show thatthe time-tracings converge at the higher frequencies whereas thetime-tracings for the noise events as shown in FIG. 5 remain more orless parallel throughout the frequency range. Also, the submarinetime-tracings of FIG. 4 bend or tilt toward the right, while the noisetracings again remain more or less straight and parallel. In addition,it can be seen that the separation between adjacent dashed and solidlines for the noise is greater than the separation between thesuccessive lines for the signals.

Accordingly, the second unique feature which provides evidence formaking a decision on the presence or absence of a submarine is that thecharacteristics of the time-tracings of the peaks and troughs for thesubmarine signals differ substantially from those of the signal-likebackground noise.

FIGS. 4 and 5 also illustrate the first feature of the signaturerecognition as pointed out with reference to FIGS. 1 and 2; that is, forsubmarine signals the su prema fall in channels 4 or 5 or both, thesuprema being indicated in FIG. 4 by the circle on filter channel 4 andthe suprema in noise events as indicated in FIG. 5 consistently appearin channel 2 as illustrated by the circle on filter channel 2.

The third unique feature which provides evidence for making a decisionon the presence orabsence of a submarine is that the ratio formed by thecentral lobe amplitudes of filter channels 1 and 7 has a range of valuesfor submarine signatures that differs substantially from that for thegeologic noise. For example, when a MAD record is impressed on thefilter bank 12, the amplitude of the central lobes in the outputs offilter channels 1 and 7 have considerably different values. Thesedifferences are best illustrated with reference to FIGS. 1 and 3.

Therefore to establish the third unique feature of submarinerecognition, the central lobe amplitudes in each filter output (for thesignals and noise events) are plotted against filter center frequency inFIGS. 6a and 6b. As can be seen from the waveforms 61 and 62 of FIG. 6awhich represent the central lobe amplitudes for MAD records 31 and 32respectively, the channel l and channel 7 amplitudes are approximatelythe same. On the other hand, waveform 64 of FIG. 6b shows that thegeologic noise events for channel 1 has a significantly higher amplitudethan channel 7.

By dividing the amplitude of channel 1 by that of channel 7, a number isobtained that is called the shape factor. Using the relative amplitudesof the central lobes in channels 1 and 7 for the signal and noiseevents, it can be seen that the shape factor for a submarine signal isconsiderably different from that of the geologic noise events. Inparticular, for the examples illustrated in FIGS. 6a and 6b, the shapefactor for the submarine signals is about 1.3 for waveform 61 and 0.78for waveform 62. For the noise event, the shape factor is approximately7.0. Accordingly, the shape factor shows a clear distinction between thesubmarine signals and the geologic noise.

From analyzing various MAD records containing submarine signals in anoise environment, it has been found that the shape factors forsubmarine signals cluster about the value of 1.0 and do not exceed 2.25,whereas for the noise signals the shape factors do not fall below 3.5.

Having thus described the three unique features which provide evidencefor making a decision on the presence or absence of a submarine signal,the following examples will illustrate the usefulness of these featuresin separating submarine signals from geologic noise.

First, consider the MAD record 31 as illustrated in FIG. 4. Thetime-frequency analysis in the lower trace reveals that all thedistinguishing submarine features are present: (a) feature 1 thesupremum is located in channel 4. (b) feature 2 the time-tracings of thepeaks and the troughs of the lobes converge at the higher frequencychannels, the traces bend toward the right and the separation betweenadjacent timetracings of the peaks and troughs is small. (0) feature 3the shape factor is approximately 1.3, which is well within the limitsfor submarine signatures.

As a second example, consider the MAD record 41 as illustrated in FIG.5. The general shape of the waveform 41 is very similar to that of waveshape 31, however, it will be shown that the wave shape 41 represents ageologic noise event and not a submarine signal. Considering again thetime-frequency analysis in the lower trace of FIG. 5, this recordindicates features possessed only by geologic noise: (a) feature 1 thesupremum is located in channel 2 rather than in channels 4 or 5. (b)feature 2 the traces remain more or less straight and parallel insteadof converging at the higher frequency channels, the traces do not bendtoward the right but again rem'ain more or less straight and paralleland the separation between adjacent time-tracings of the peaks andtroughs is large rather than small, (c) feature 3 the value of the shapefactor as illustrated in FIG. 6b is approximately 7, which is in therange of geologic noise. It may, therefore, be concluded that the eventis not a submarine signal but rather geologic noise.

As a final and more challenging example of the accuracy with which thepresent system is capable of distinguishing submarine signals from ahigh noise environment, consider the MAD record 32 as illustrated inFIG. 4 wherein the submarine signal is completely undetectable by thehuman eye.

Looking again at the time-frequency analysis in the lower trace, thedistinguishing features of a submarine signal will now be investigated:(a) feature 1 the channel location of the supremum is not clear-cutoccurring in either channel 2 or 6; hence, no conclusion can be drawnconcerning the presence of a submarine signal from this particularfeature. (b) feature 2 the characteristic convergence and smallseparation are present, but the trace is erratic, making this evidenceinconclusive. (c) feature 3 despite the uncertainty of features 1 and 2,when the shape factor which is approximately 0.8 is considered, it isfairly conclusive that the MAD record 32 contains a submarine signature.

It can be readily appreciated from the above description of theoperation of a specific embodiment of the invention that a distinctioncan be made between submarine signals and background transient noiseeven in high transient noise conditions; accordingly, by extracting theunique characteristics of a submarine signal from the signal-likebackground noise, it is possible for an operator to recognize submarinesignatures previously undetectable.

It should be understood, of course, that the foregoing disclosurerelates to only a specific embodiment of the invention and that numerousmodifications or alternatives may be made therein without departing fromthe spirit and the scope of the invention as set forth in the appendedclaims.

I claim:

1. An apparatus for detecting submarine signatures in a signal-likenoise background (as recited in claim 3 wherein said plurality of low-Qband pass filters further comprises) comprising:

means for receiving magnetic anomaly detection signals in thetime-domain, said means including a magnetic anomaly detector fordetecting variations in the earths magnetic field caused by the presenceof a submarine;

separator means electrically connected to said detector means forseparating said signals into preselected frequency components, saidseparator means including a plurality of low-Q band pass filters havingtheir inputs connected to said magnetic anomaly detector for separatingsaid signals in the time-domain into preselected frequency components inthe frequency-time domain, said plurality of (seven) band pass filtershaving center frequencies substantially of 0.048 cycles per second,0.075 cycles per second, 0.12 cycles per second, 0.19 cycles per second,0.30 cycles per second, 0.48 cycles per second and 0.75 cycles persecond(.); converter means connected to the outputs of each of saidfilters for converting said frequency components into signals havingunidirectional lobes; and display means operatively connected to saidconverter means for displaying said signals having unidirectional lobes.

1. An apparatus for detecting submarine signatures in a signallike noisebackground (as recited in claim 3 wherein said plurality of low-Q bandpass filters further comprises) comprising: means for receiving magneticanomaly detection signals in the time-domain, said means including amagnetic anomaly detector for detecting variations in the earth''smagnetic field caused by the presence of a submarine; separator meanselectrically connected to said detector means for separating saidsignals into preselected frequency components, said separator meansincluding a plurality of low-Q band pass filters having their inputsconnected to said magnetic anomaly detector for separating said signalsin the time-domain into preselected frequency components in thefrequency-time domain, said plurality of (seven) band pass filtershaving center frequencies substantially of 0.048 cycles per second,0.075 cycles per second, 0.12 cycles per second, 0.19 cycles per second,0.30 cycles per second, 0.48 cycles per second and 0.75 cycles persecond(.); converter means connected to the outputs of each of saidfilters for converting said frequency components into signals havingunidirectional lobes; and display means operatively connected to saidconverter means for displaying said signals having unidirectional lobes.